It has been reported that turkey erythrocytes contain a guanosine triphosphatase (GTPase) with a very low Km for GTP that can be activated by catecholamines. This activated GTPase is inhibited by cholera toxin. To evaluate the hypothesis of Cassel and Selinger that this GTPase is a regulatory moiety of the adenylate cyclase complex, we are examining in parallel the effects of prostaglandins, catecholamines, toxin, and other agents on GTPase and adenylate cyclase activities in human mononuclear cell membranes and turkey erythrocyte membranes.